A central problem in microbial pathogenesis is how cells of the immune system integrate multiple signals to induce an appropriate response and how pathogens avoid and/or manipulate the host response. Listeria monocytogenes as a highly tractable model intracellular pathogen with which to approach this problem. Using wild-type and mutant bacteria lacking a secreted pore-forming hemolyisn (LLO), two separate pathways were discovered leading to host gene expression: a vacuolar pathway characterized by Toll-like Receptor signaling and a cytosolic pathway characterized by Interferon beta (IFNb) production. IFNb expression was mediated, at least in part, by L. monocyogenes autolysins (p60 and NamA) secreted by the recently discovered SecA2 pathway. It appeas as though L. monocytogenes is manipulating a host system of innate immunity by the production of a specific peptidoglycan (PGN) cleavage product. In Aim I, the macrophage transcriptional response to vacuolar and cytosolic bacteria will be determned using wildtype L. mononcytogenes and LLO-minus mutants in combination with B. subtilis or B. subtilis expressing LLO. Using macrophages isolated from mice lacking the IFNabR, MyD88, or TLR2, a panel of genes representing the vacuolar and cytosolic signaling will be established. In Aim II, the bacterial autolysins affecting macrophage signaling will be determined by introduction of in-frame deletions and characteization of the resulting mutants in mice and macrophages from wild-type and NOD1 and NOD2 knockouts. In Aim III, the contribution of specific PGN fragments will be determined using reverse phase high pressure liquid chromatography and mass spectrometry combined with introduction of purified PGN fragments into cells using liposomes with and without LLO. In Aim IV, activated peritoneal macrophages will be used to test the hypothesis will be tested that the cytosolic pathway actually detects material (PGN) derived from a vacuolar compartment resulting from degradation in a phagosome. In the final aim, the composite transcriptional 3rogram induced by L. monocytogenes will be compared with that induced by M. tuberculosis, F. tularensis and H. capsulatum.